So on my new m&p9 I can't use the pulling the slide back technique. I've tried and it won't slide forward unless I disengage the slide stop. Mine is super stiff as well hence why I tried pulling the slide but for some reason it won't ram home without pressing the slide catch.
If the magazine is empty you have press the slide release. The button should be stiff. If you put in a loaded magazine you should be able to just pull the slide rearward and let go. The slide will slam forward loading the next round.